Power harvesting relates to the capture of electromagnetic energy from the environment. Examples of electronic circuits which can be powered from the environment include sensor systems that occasionally need to read a sensor value, and identifier circuits that respond to a challenge by sending out an ID, or an authentication signal.
It is usually difficult to harvest enough energy for free from the environment, other than for extremely low power circuits. In general a power signal is instead sent to the circuit to actively load it with energy. RFID tags are an example of this approach.
There are however circuits which potentially could operate based on energy harvesting, making use of a power harvesting antenna, in which the required amount of energy is very small. US20120299396A1 discloses such a power harvesting solution. In an energy harvesting system, the energy gathered by a single antenna may typically be inadequate. Hence multiple antennae can be used to increase the total harvested energy.
A power harvesting circuit typically comprises an antenna, a rectifier circuit and an energy storage capacitor.
It is known that the DC output power of known rectifier circuit designs scales quadratically with the input power at low power levels, because of the dominant 2nd order term in the Taylor expansion of the rectifier transfer function for small signals. This is explained in the paper “Modeling of RF energy scavenging for batteryless wireless sensors with low input power, by Yan Wu; Dept. of Electr. Eng., Eindhoven Univ. of Technol. (TU/e), Netherlands; Linnartz, J.-P.; Hao Gao; Matters-Kammerer, M. K. Published in: Personal Indoor and Mobile Radio Communications (PIMRC), 2013 IEEE 24th International Symposium on 8-11 Sep. 2013.
For this reason, for multiple antenna systems it is not attractive to rectify the power from each antenna individually and then sum the energy from all branches. Instead, it is more effective first to combine the antenna signals and then to rectify.
However, just combining the signals may lead to the undesirable situation that for some phase angles at which the power comes in, the signals at the antennae will cancel each other. It is unavoidable that such cancellation can occur.
FIG. 1 shows a known energy scavenging receiver. It consists of a receive antenna 10, a matching network 12 that maximizes power transfer from the antenna, an RF to DC rectifier 14 and an energy storage device 16 such as a capacitor or rechargeable battery that that stores the energy. A crucial part of the receiver is the RF-DC rectifier 14. One example shown in FIG. 1 is a Dickson charge pump structure, that converts the received RF energy into a DC form suitable for storage. The basic building block of a Dickson charge pump is a Greinacher voltage doubler 18.
The input to the voltage doubler 18 is a sinusoidal wave r(t)=A sin(2πfct), where A is the amplitude and fc is the center frequency. In the ideal case, when A is large enough, the equilibrium voltage on the output capacitor C of the voltage doubler would be 2A. Since the energy stored in a capacitor is E=½CV2, the energy scavenged by the voltage doubler is proportional to A2. For applications in wireless sensors, A is usually small due to limited transmission power and large propagation loss. The stored energy is thus proportional to A4.
This fourth power is a very severe limitation to making the rectifier efficient when operating at low input powers. If the input signal A is very weak the rectifier becomes very inefficient. In fact the current versus voltage characteristic of any diode acts as almost a straight line near the zero point (V=0 Volt, I=0 Amp). Hence for very weak signals the diode acts more as a resistor with a linear V-I characteristic, and fails to have a rectifying behaviour. That rectification can only work if the V-I characteristic is large enough to experience the higher order non-linear terms of the V-I characteristic.
For small voltages, significantly below the threshold the diode current can be modelled according to a second order series expansion:I=aV+bV2 
There has been a lot of research into designing power harvesting circuits in such a way that they can handle very high frequencies (even as high as several or several tens of GHz) and very low input voltages.
This invention is aimed at the problem of harvesting energy from multiple antennae.
There are known circuits which combine antenna signals, for example beam steering and maximum ratio combining systems used in communications and radar systems. These techniques aim at improving the signal strength by phase shifting the signals from all antennae such that they are in phase and add coherently, that is constructively, such that the voltages add up. In a maximum ratio combining system, not only the phases but also the amplitudes are adjusted to optimise the signal to noise ratio. For a system with multiple antennae in the presence of (only) uncorrelated additive white Gaussian noise, maximum ratio combining provides the best signal to noise ratio.
In these systems, a communications base station can exploit antenna gain and beam forming because it has adequate power for the control algorithms. However, for a harvesting application, there is no power for an adaptive circuit that guarantees coherent aggregation of power from multiple antennae with unknown phase differences. This makes it difficult to exploit beam formers and rectifiers that can adaptively and optimally merge the power from multiple differently phased inputs without requiring energy beyond the received energy.
The problem arises for energy harvesting systems that power is not available to operate a circuit to adaptively adjust the phase or amplitude of the incoming RF signals. The output power of these circuits can be less than the power that the circuit draws from its power supply, which makes them useless for energy harvesting.